Electron discharge device



Sept. 22, 1959 E. J. STERNGLASS 2,

ELECTRON DISCHARGE DEVICE Filed June 4, 1954 DYNODES PHOTOEMITTER\|5 lo/nmeer 20 F an Fig.2 Fig-3. 5 MEMBER SECONDARY EMISSIVE wk LAYERSUPPORT- MEMBERS ELECTQON SCATTEElNG 42/ k LAYER mxwo ELECTRON gscATTezms LAYER WITNESSES. INVENTUR Ernest J. Sferngloss. ,4 Y C W.

ATTORNEY Patented Sept. 22, 1959 ice ELECTRON DISCHARGE DEVICE Ernest J.Sternglass, Pittsburgh, Pa., assignor to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of PennsylvaniaApplication June 4, 1954, Serial No. 434,467

11 Claims. (Cl. 313-68) This invention relates to electron dischargedevices, and more particularly to those devices having secondaryelectron emissive electrodes.

It is an object of this invention to generate secondary electrons on theside opposite to which primary electrons strike thin films of secondaryelectron emissive material.

It is another object to provide improved efliciency of secondaryelectron emissive electrodes or dynodes.

It is another object to provide a stable secondary electron emissivesurface for operation over a long period of time.

It is another object to provide an improved secondary electron emissiveelectrode that does not require complicated activation procedures.

It is another object to provide a secondary electron emissive electrodethat may be exposed to air and other gases without deterioration.

It is another object to provide a secondary electron emissive, electrodethat permits out-gasing of the enclosing envelope at elevatedtemperatures without destroying the secondary emissive electrode.

It is another object to provide a secondary electron emissive electrodethat may be operated at elevated temperatures without destroying itssecondary emissive properties.

It is. another object to provide a secondary electron emissive electrodehaving a zero-signal dark current.

It is another object of my invention. to provide a device for themultiplication of electron images without appreciable loss ofinformation in the spatial distribution of the original electron image.

These and other objects are effected by my invention as will be apparentfrom the following description taken in accordance with the accompanyingdrawings, in which:

Figure l is a diagrammatic view of an electron multiplier tube embodyingmy invention;

Fig. 2 is a sectional view of a portion of a secondary missiveelectrode, utilized in the device shown in Fig. 1; and

Fig. 3 is a front view of the structure shown in Fig. 2.

Referring now to Fig. 1, an electron discharge tube is shown comprisingan envelope 9 having a cylindrical portion 10 and end plates 13 and 14positioned at opposite ends thereof. A planar cathode or electronemissive surface 15 is positioned near to or on the end plate 13 and ananode or target element 16 is positioned at the opposite end of theenvelope 9 near to or on the end. plate 14. A plurality of the secondaryelectron emissive electrodes or dynodes 17, 18 and 19 are interspaced.between the cathode 15 and target 16. The planar electron emissivesurface 15 positioned near the end plate 13 may be of any suitable typesuch as thermionic or photoemissive. In my specific embodiment, aphotocathode planar surface 15 is utilized as the source of electronsfor the discharge device.

The photocathode surface 15 may be of a suitable material, such ascaesium antimony, capable of emission of electrons upon lightimpingement. The end plate 13 is of a transparent material such as glassso as to permit passage of light. The photo-emitting surface 15 may bemounted on a suitable supporting transparent conductive surface or maybe deposited on the interior surface of the end plate 13. It isdesirable in most cases to provide a conductive coating 21 on the endplate 13 prior to the depositing of the photocathode material so as toobtain an electrode for the photo-emitting surface. A suitabletransparent conductive coating may be of a material such as Nesa.

The target electrode 16 is positioned at the opposite end of theenvelope 9 near the face plate 14. The target 16 may be of any electronsensitive material so as to develop a signal representative of theelectron bombardment. In my specific embodiment a phosphor screen isutilized as the target electrode 16 of the electron discharge device.The phosphor screen 16 is of a suitable material such as zinc sulphidewhich may be placed on a suitable transparent conductive supportingmember or deposited on the end plate 14. If the phosphor screen 16 isdeposited on the end plate 14 as shown in my specific embodiment, it isdesirable that a transparent conductive layer 20 such as Nesa bedeposited on the end plate 14 prior to the'phosphor screen 16 to serveas an electrode. The phosphor screen 16 may also be deposited directlyon the end plate 14 if desired and a thin electron permeable conductivelayer such as aluminum be deposited upon the exposed surface phosphorscreen to serve as the voltage electrode. The aluminum backing will alsoenhance the light output of the phosphor screen.

Positioned between the cathode 15 and the image screen 16 are aplurality of secondary electron emissive electrodes or dynodes 1'7, 18and 19 and by way of example, I have shown only 3 dynodes. The number ofdynodes Within the envelope 9 is dependent on the amount ofamplification desired from the device and a single dynode may besufficient in some applications.

The requisite potential for the electrodes within the envelope 9 may beobtained from a potentiometer, or any other suitable device. in thespecific device shown in Fig. l, I have utilized a battery 22 having itsnegative terminal connected by means of a conductor 24 to the conductivelayer 21. to supply a potential to the cathode 15 and the positiveterminal of the battery 22 is connected by means of a conductor 25 tothe conductive layer 20 to supply a potential to the image screen 16. Aplurality of resistors 31, 32, 33, and 34 of equal value connected inseries are connected across the conductors 24 and 25 so as to be shuntedacross the battery 22. A lead 39 is provided from the first dynode 17 toa point between resistors 31 and 32 while the second dynode 18 isconnected to a point between the resistances 32 and 33, and the thirddynode 19 is connected to a point between the resistors 33 and 34. Thefree end of resistor 31 is connected to the conductor 24 while the freeend of resistor 34 is connected to conductor 25. In this manner, thesuccessive electrodes 17, 18, 19 and 16following the cathode 15'haveprogressively increasing steps of positive potential with respect to thecathode 15 so as to accelerate the electrons from electrode toelectrode. Although I have shown equal steps of voltages between theelectrodes 17, 18, 19 and 16, it may be desirable to operate the imagescreen 16 at a substantially higher voltage than the other electrodes17, 18 and 19.

A light image may be projected onto the cathode surface 15 by anysuitable means so as to activate the photoemitting cathode 15. By way ofexample, I have shown a kinescope 28 with a suitable lens system 29between the kinescope 28 and a photocathode surface 15 for purscope ontothe photocathode surface 15.

Referring to Figs. 2 and 3 for the detailed structures of the dynodes17, 18 and 19 shown in Fig. 1, I have shown a portion of a dynode forpurposes of illustration. The dynode structure is comprised of at leasta secondary electron emissive layer 40. The secondary emissive layer 40is of a crystalline insulator material such as an alkali-halide (Forexample KCl or NaCl) which has the property of allowing the flow ofsecondary electrons within the material for a long distance before beingabsorbed. It has also been found that the higher the atomic number thehigher the emission, for example cesium iodide has a high average atomicnumber. The term average atomic number as used herein refers to theatomic number of the element or the average of the atomic numbers of theelements in a compound. An alkaline earth oxide is also a suitablesecondary emissive material. The secondary electron emissive layer 40 isof a thin planar sheet having substantially the same area as thephotocathode surface 15 and parallel thereto. The thickness of thesecondary emissive layer is of the order of 100s to 1000s of angstromunits, or 10 10 cm.

The secondary electron emissive layer 40 may be deposited on an eventhinner layer 41 of a high atomic number material such as gold oruranium. The thickness of this heavy metal layer is on the order of 100angstroms or less. The function of the heavy metal film 41 is to aid inscattering the incident electrons so that the electrons entering thesecondary electron emissive layer 49 will be at an angle with theincident electrons trajectory which is normal to the surface of thelayer 40 thereby enhancing the secondary emission of the secondaryemissive layer on the side opposite to that on which the primaries areincident. The heavy metal layer 41 is in turn supported by a fine meshgrid 42. The grid 42 in the preferred embodiment of the device isfabricated from a thin sheet of conducting material such as copper ornickel. The metal grid 42 may then be pleated or coated if desired withan inert metal such as gold or platinum in order to insure greaterresistance of the grid 42 to oxidation and corrosion. The holes orapertures 43 in the grid 42 may be etched in a sheet of suitablematerial so as to provide a large open area screen of about 70 to 90%.In my specific embodiment, the sides 44 of the apertures 43 are taperedtoward the cathode 15. The grid 42 may also be considered as a cellularor honeycomb structure with the sides 44 of the cells or apertures 43tapered towards the source of incident electrons. By designing the grid42 in this fashion, many of the incident electrons that would be lost inconventional type grids are scattered by the sloping or tapered sides44- of the apertures 43 in the grid member 42 so as to produce secondaryelectrons in the secondary electron emissive layer 40. The tapered griddesign permits the grid 42 to be made mechanically quite strong bymaking the walls 44 of the apertures relatively thick near the heavymetal layer 41 and relatively thin at the opposite side of the grid 42and thereby still retain a large open area screen of transmission. In myspecific device to obtain the desired resolutions in an imageintensifier, it is desirable to have on the order of 500 apertures perinch of screen. Fewer apertures per unit distance may be used when noimaging is desired.

The dynodes 17, 18 and 19 may be constructed by suitable methods knownin the art. For example, an organic film such as nitrocellulose lacqueris settled on the grid structure by covering the grid with water andapplying the organic solution with solvent on the surface of the water.As the organic solution spreads out on the surface of the water thesolvent evaporates leaving the organic film. The water is then removedallowing the film to settle on the grid. The organic film is dried andthe heavy metal film, if used, is evaporated onto the free .4 surface ofthe organic film. The secondary electron emissive layer is thenevaporated onto the organic film or the heavy metal film (if used). Ithas been found desirable to use the heavy metal film with thealkalihalide of lower average atomic number while it may be omitted withthe alkali-halides of higher average atomic number. The higher averageatomic number alkali materials sufficiently scatter the incidentelectrons while the lower atomic numbered require the heavy metal layer.The organic film may then be baked olf leaving the heavy metal layer onthe grid and the secondary electron layer upon the heavy metal layer.This is only one of many methods of depositing the layers on the grid.The etched-foil type of mesh for the grid 42 is preferred to the wovenmesh structure because of a fiat surface available for supporting thethin layer of secondary electron emissive material.

Another method of construction is to deposit the crystalline layer 40 onto a permanent film of a suitable material such as SiO of thicknessequal to tens of angstroms previously deposited by techniques similar tothat described above for the organic film. It is also possible, if thevoltage between dynode structures is sufficiently high, to use aself-supporting thin metallic foil instead of the supporting grid 42 andthe secondary electron emissive layer 40. Also the dynodes may bemounted at different angles to the direction of the incident electrons,while at the same time the secondary electron emissive layer may be madethinner, so as to cause incident electrons to have larger angles ofincidence with the dynode so that secondaries form close to the surfaceof the crystalline layer 40.

If a greater mechanical strength is desired in the dynode, a second gridmay be placed in coincidence with the first grid but on the oppositeside of the secondary electron emissive layer. Such'a double-gridarrangement has the further advantage of reducing the undesirableemission of electrons at large angles relative to the normal surface. Italso may be desirable in some cases to evaporate the secondary electronemissive material over the sides of this second grid supporting mesh inorder to further enhance the ratio of secondary electron emission toincident electrons within the dynode structure.

In the operation of the device shown in Fig. 1 an image is projected bythe kinescope 28 through the optical means 29 upon the photocathodesurface 15. The photocathode layer 15 in response to the light imageprojected thereon will generate an electron image representative of thelight image projected thereon. Under the influence of the potentialapplied to the first dynode 17 the electron image emitted from thephotocathode layer 15 will be accelerated to a sufficient velocity tothe ingressive side of the dynode 17, such that the incident electronsin passing through the secondary emissive layer 40 will be reducedsubstantially to Zero. As previously described, substantially all of theincident electrons, striking the grid 42 and not passing directlythrough the grid aperture 43 will be scattered or diffused by thetapered walls 44 into the secondary electron emissive layer. It has beenfound that the number of secondary electrons emanating from the emissiveside of the secondary electron emissive layer 40 is many times greaterthan the number of impinging primary electrons. In a typical layer ofabout 300 angstrom units of KCl deposited on 40 angstrom units of gold,at an incident energy of 24 kilovolts between 4 to 7 secondary electronswere found to be emitted for each primary electron striking the layer40. Consequently, an electron multiplication is obtained in the primarycurrent obtained from the photocathode surface 15 by the first dynode17. The secondary emissive electrons released from the first dynode 17are accelerated to the second dynode 18 where this procedure is againrepeated. The secondary electron emission from the emission side of thesecondary emission layer 40 of the second dynode 18 is many timesgreater than the incident electron thereon. Similarly, the

secondaryelectrons released from the second dynode 18 are accelerated tothe third dynode 19, where.-.again the electron current is amplified andfurther multiplication occurs. The electrons emitted fromthe-thirddynode 19 are accelerated to the phosphor screen 16 where anenhanced light image is obtained corresponding .to the light imageprojected on the photocathode 15.

By utilization of the grid structure as previouslydescribed and byvirtue of the large number ,of apertures per unit area, the dispersionof the electron image flowing between the photocathode and theimagescreen 16 .is limited to a small amount so that substantially .noreduction of details is lost from the original light-image .projectedthereon. it has also beenfound that aclose spacing of the order of a fewtenths of an inch betweendynode members 17, it; and 19 also aids ininsuring that a satisfactory picture is obtained on the image screen 16without the aid of electromagnetic focussing. It has been found that bycontrolling the accelerating voltages .between dynode stages 17, 18 and19 so thattheincident electrons are not able to completely penetratethesecondary electron emissive layer 40, an excellent image is obtainedon the screen 16.

Although it has long been realized that secondary electron yield fromvarious simple insulating material such as an alkali halide for example:potassium chloride and .calcium chloride is large, certain .practicalobstacles have been in the way of actual utilization ofsuch materials inelectron multipliers. It has previously been found that even a very thinlayer of the order of .10 to 100 atoms deposited on a heavy metalbackingcharges up under electron bombardment when used as .a simplefront surface secondary electron emitter. This results injsecondaryemissive yields that depend critically on the beam current and thethickness of the layer. The resulting instability has made itimpossible, ,prior to this time, to build a workable device using alkalihalide as a secondary emitting substance in electron multipliers.

I have found that these obstaclesmay be surmounted and a satisfactorytransmission type. secondary electron multiplier dynode structure may beconstructed. ,Lhave found that if the electron beam completelypenetrates the alkali halide layer or more precisely, when it penetratessuch as to produce slow electrons capable of carrying a currentthroughout the body of the thin insulating alkali halide layer, thecharging-up and instability are avoided. Both the metallic supportinggrid and thevernployment of high energy electrons are instrumental .in.bringing. about the desired effect in that the grid serves to reduce theconduction path for the replenishment of electrons, and the high energyelectrons serve to reduce the electrical resistance by providingconduction electrons throughout the insulating layer. It should be alsonoted that it is necessary to use as pure and simple a crystallinematerial as possible in which the secondary electrons can travelrelatively large distances and therefore escape from much greater depthsthan the case of metals, complex cesiated or activated metallic layer orinsulators of an amorphous structure such as glass. The provision of avacuum between successive amplification stages allows the accelerationof the slow secondary electrons coming out of the emissive side of theprevious dynode. Furthermore, it is important that there be excellentinsulation between stages so as to avoid large leakage current thatwould swamp out any signal current or even in extreme cases, destroy thelayer by the large heating produced.

The utilization of an alkali-halide and grid supporting structurerequires no special activation procedures nor is it affected by exposureto air unlike the complex surfaces such as cesiated silver presentlyemployed for dynode surfaces. The alkali-halides and the gold or similarmetals also possess a high melting point and high work-function and,therefore, do not suffer from most of the disadvantages of presentlyused complex secondary emitters. It has been found that these featuresare particularly important for applications to low signal-leveloperation in that a'greatly reduced thermionic and photoelectricemission as well as leakagecurrent is obtained between stages. The lowleakage current between stages results from theabsence of caesium orsimilar vapors liberated in the forming of the secondary electronemission dynodes together with the greatly increased sensitivity at lowtemperatures inherent in the use ofa pure crystalline insulator, adeviceincorporating the structure described above gives a superior andpractical dynode structure.

This type of secondary emissive surface is also suited to problems oflow-level electron-image amplification such as in infrared and X-rayimage tubes because of the small loss of definition .to be expectedresulting frornthe extreme thinness of the multiplying surfaces and theabsence of fast stray electrons. Since the current densi tiesencountered in this type of application are extremely low, about 10*amp/cm. or less, and any possible deterioration of the crystallinesurfaces as a result of bombardment is a function only of the totalcharge involved, the life of such a multiplying surface can be shown tobe large compared to minimum requirements. Thus, I have obtained asensitive electron multiplier dynode which permits the use of strongmetallic supports for an extremely thin secondary electron emissivesurface, theabsence of any activation in the completed tube, thehandling of the dynodes in open air, and the outgassing at temperaturesas high as normally used in vacuum tubes.

Although I have shown the possible utilization of a heavy metal layer 41in the structure shown in Figs. 2 and 3, I have also found that byincreasing the thickness of the secondary electron emissive layer 46that the dynode will operate at substantially the same efiiciency.

In one model that I have built utilizing only 50% open mesh screen, ayield of greater than 3 secondary electrons was obtained from the dynodestructure with one incident electron. The incident energies utilized inthis device were ofthe order of 1500 volts.

While I have shown my invention in only one form, it will be obvious tothose skilled in the art that it is not so limited, but is susceptibleof various other changes and modifications without departing from thespirit and scope thereof.

I claim as my invention:

1. An electron multiplier comprising, anenvelope and having therein acathode, a target and a. plurality of dynodes positioned between saidcathode and said target, said dynodes characterized in having a thinsecondary emissive layer of insulating material having an ingressivesurface and an emissive surface with said ingressive surface facing saidcathode, a supporting member for said secondary emissive layercontacting said ingressive surface, said supporting member having aplurality of cellular openings.

2. An electron discharge device comprising an envelope having therein aplanar cathode positioned at one end thereof, and a target electrodenear the opposite end of the envelope and a plurality of dynodespositioned between said cathode and said target, each of said dynodesbeing capable of producing secondary electrons transmissively at a ratiogreater than unity and comprising a thin layer of secondary electronemissive insulating material of about angstroms in thickness supportedon a metallic structure.

3. An electron multiplier comprising an envelope and having therein aplanar photo-emissive cathode positioned near one end of said envelope,a planar electron responsive electrode positioned at the opposite end ofsaid envelope, a plurality of dynode structures positioned between saidcathode and said electron-responsive electrode, said dynodes comprisinga thin secondary emissive layer of insulating material of the order of100 angstroms in thickness and a thin metallic coating.

4. An electron multiplier comprising an envelope and having locatedtherein a planar photo-emissive cathode, an electron-responsiveelectrodepositioned at the opposite end of said envelope with respect to saidcathode, a plurality of planar dynode structures positioned between saidcathode and said target, said dynode comprising a thin layer ofcrystalline insulating material and means for supporting said layer.

5. An electric discharge device comprising an enve lope and havintherein, a planar photo-emissive cathode positioned at one end of saidenvelope, an electron responsive planar target positioned at theopposite end of 'said envelope with respect to said cathode, and aplurality of planar dynodes positioned between said cathode and saidtarget, each of said dynodes being capable of producing transmissivesecondary electrons at a ratio greater than unity and comprising a thinlayer of alkali halide material supported on a perforated metallic gridstructure.

6. An electric discharge device comprising an envelope and havingtherein a planar cathode positioned at one end thereof, a targetelectrode positioned near the opposite end of said envelope and aplurality of dynodes positioned between said cathode and said target,each of said dynodes comprisin a thin layer of pure crystallineinsulating material exhibiting the properties of producing secondaryelectrons transmissively and of increasing conductivity upon electronbombardment supported on a metallic grid-like structure.

7. An electron multiplier comprising an envelope and having therein, aplanar cathode and electron responsive target positioned at the oppositeend of said envelope with respect to said cathode, a plurality of saiddynodes positioned between said cathode and said target, each of saiddynodes comprising a thin layer of secondary emissive insulatingmaterial supported on a metallic planar grid-like member, said grid-likemember having a plurality of cellular openings therein with the sides ofsaid openings tapered toward said cathode.

8. A transmissive dynode structure for an electron multiplier comprisinga thin layer of a crystalline insulating material characterized inexhibiting the properties of producing transmissive secondary electronsat a ratio greater than unity and of an increasing conductivity uponelectrode bombardment, a layer of high average atomic number materialdeposited on the bombardment side of said insulating layer to scatterbombarding electrons and means for supporting layers. l V

9. An electron multiplier comprising an envelope and having therein aplanar photo-emissive cathode, a planar electron responsive targetpositioned at the opposite end cathode, each of said dynodes comprisin athin continuous layer of insulating material mounted on a gridlikemetallic supporting structure, means positioning said dynodes inrelatively closed spaced relationship, means to focus the electrons fromsaid cathode on the first of said dynodes, means for focusing thesecondary electrons from the last dynode upon the electron responsivetarget and means for maintaining said electrodes at increasinglypositive potentials with respect to said cathode.

10. A secondary emissive dynode structure comprising a continuous layerof insulating material capable of the emission of secondary electronsfrom the opposite surface on which the bombarding electrons impinge, anda continuous layer of a conductive material of a higher average atomicnumber than said insulating material on the bombardment side of saidinsulating material for scattering the bombarding electrons into saidinsulating layer.

11. A transmissive type secondary electron emissive dynode structurecomprising a continuous layer of insulating material capable of theemission of secondary electrons from the opposite surface with respectto the surface which is bombarded by primary electrons, said insulatingmaterial having a large energy gap between the filled valence band andthe conduction band so that secondary electrons within the layer cantravel relatively large distances and escape therefrom, electricalconductive means provided on the surface of said layer on which saidbombarded electrons impinge to reduce the conduction path forreplenishment of electrons over the emissive surface of said insulatinglayer.

References Cited in the file of this patent UNITED STATES PATENTS2,196,278 Teale Apr. 9, 1940 2,254,128 Van Den Bosch Aug. 26, 19412,254,616 McGee Sept. 2, 1941 2,254,617 McGee Sept. 2, 1941 2,527,981Bramley Oct. 31, 1950 2,739,084 Sommer Mar. 20, 1956 OTHER REFERENCESPhoto-Electric Multipliers," S. Rodda, 1953, Macdonald & Co., Ltd.,London. Especially pages 20 and 21.

